The present invention relates to a process for continuously preparing a polymer by free-radical polymerization, in which at least three feedstocks are mixed in one or more mixers with microstructures and then polymerized in at least one reaction zone.
WO 99/54362 describes a process for continuously preparing polymers, in which at least two reactants are conducted through and combined by a micromixer, mixed with one another and then polymerized in a tubular reactor. In this case, the starting materials are preheated to such an extent that they reach the required reaction temperature after entering the micromixer. This procedure is intended to prevent the formation of polymers with a bimodal molar mass distribution and high molecular weight fractions which can lead to blockage or coverage of the reactor system. Not only is this procedure energy-intensive, but an undefined preliminary conversion in the mixer also has to be expected. In addition, in the case of monomers containing acid groups and at the high reaction temperatures as are required for preparation of low molecular weight products, an increased degree of corrosion problems occurs. Moreover, even a brief stoppage in a feed or a decline in the flow can already result in polymerization in the mixer owing to the preheating, and hence also in blockage of the micromixer.
WO 03/037501 discloses a process for (co)polymerizing olefinically unsaturated monomers, in which at least two starting materials in the form of two liquid streams in opposite directions are mixed in a mixer with intermeshing microchannels before they enter a reaction vessel. Owing to the channel dimensions used with cross sections in the range from 10 to 100 μm, there is the risk of coverage or blockage of mixer and/or reactor in this process too. This is problematic since apparatus with microstructures is generally also not very accessible for cleaning purposes.